I. Introduction
Water is the essence of life and an important nutrient for human, animals, plants, and environment. However, contaminated water posed a significant threat in almost every aspect in China. One of the most severe water pollution is eutrophication that is often caused by too many nutritive salts such as nitrate and phosphate. Real-time monitoring for the content level of nutritive salts is the best choice to prevent and reduce the harm, and planar electrode sensor is a promising tool for detection and measurement of ion concentration [1]–[3].